Trending doesn't seem to follow the user's setting "Posts order." I don't know if this is intentional or the sort order is based off some magic trending algorithm.
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Trending is sorted by popularity – i.e. by the number of likes of each post.
Derek, thanks for your questions.
Trending section only contains posts published during past 3 days, and it gets recalculated hourly. So, new posts get added to it and expired posts get evicted gradually.